A package on package structure includes a first substrate having a first region and a second region, a bump formed on the first region of the first substrate, a first semiconductor die bonded to the second region of the first substrate, and a semiconductor die package bonded to the first substrate. The bump includes a metallic structure and a plurality of minor elements dispersed in the metallic structure. The semiconductor die package includes a connector bonded to the bump, and the first semiconductor die is between the semiconductor die package and the first substrate.